# Difference between revisions of "First derivative test"

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===What the test says: combined sign versions=== | ===What the test says: combined sign versions=== | ||

− | Suppose <math>f</math> is a [[function]] defined around a point <math>c</math> (i.e., <math>f</math> is defined in an [[open interval]] containing <math>c</math>) and is continuous at <math>c</math> | + | Suppose <math>f</math> is a [[function]] defined around a point <math>c</math> (i.e., <math>f</math> is defined in an [[open interval]] containing <math>c</math>) and is continuous at <math>c</math>. We do not care whether <math>f</math> is differentiable at <math>c</math>; however, the test makes sense only if <math>f</math> is differentiable on the immediate left and immediate right of <math>c</math>. |

Then, we have the following (we list only the ''strict'' cases in the table below): | Then, we have the following (we list only the ''strict'' cases in the table below): | ||

{| class="sortable" border="1" | {| class="sortable" border="1" | ||

− | ! Sign of <math>f'</math> on immediate left of <math>c</math> !! Sign of <math>f'</math> on immediate right of <math>c</math> !! Conclusion about local minimum, local maximum, or neither | + | ! Continuity and differentiability assumption !! Sign of <math>f'</math> on immediate left of <math>c</math> !! Sign of <math>f'</math> on immediate right of <math>c</math> !! Conclusion about local minimum, local maximum, or neither |

|- | |- | ||

− | | positive || negative || strict local maximum | + | | <math>f</math> is continuous at <math>c</math> and differentiable on the immediate left and immediate right of <math>c</math> || positive || negative || strict local maximum |

|- | |- | ||

− | | positive || positive || neither local maximum nor local minimum | + | | <math>f</math> is continuous at <math>c</math> and differentiable on the immediate left and immediate right of <math>c</math> || positive || positive || neither local maximum nor local minimum |

|- | |- | ||

− | | negative || negative || neither local maximum nor local minimum | + | | <math>f</math> is continuous at <math>c</math> and differentiable on the immediate left and immediate right of <math>c</math> || negative || negative || neither local maximum nor local minimum |

|- | |- | ||

− | | negative || positive || strict local minimum | + | | <math>f</math> is continuous at <math>c</math> and differentiable on the immediate left and immediate right of <math>c</math> || negative || positive || strict local minimum |

|} | |} | ||

If we replace ''positive'' by ''nonnegative'' and ''negative'' by ''nonpositive'' in the rows corresponding to strict local maximum and strict local minimum, we could potentially lose the ''strictness''. | If we replace ''positive'' by ''nonnegative'' and ''negative'' by ''nonpositive'' in the rows corresponding to strict local maximum and strict local minimum, we could potentially lose the ''strictness''. | ||

+ | |||

+ | ===Relation with critical points=== | ||

+ | |||

+ | The typical goal of the first derivative test is to determine whether a [[critical point]] is a point of local maximum or minimum. Hence, the test is typically applied to critical points. ''However, when applying the first derivative test, we do not need to check whether the point in question is a critical point. In other words, if the condition for being a point of local maximum or minimum is satisfied, then the point in question is automatically a critical point and this condition need not be checked separately.'' | ||

===Succinct version=== | ===Succinct version=== |

## Revision as of 16:19, 24 April 2012

## Contents

## Statement

### What the test is for

The **first derivative test** is a partial (i.e., not always conclusive) test used to determine whether a particular critical point in the domain of a function is a point where the function attains a local maximum value, local minimum value, or neither. There are cases where the test is *inconclusive*, which means that we cannot draw any conclusion.

### What the test says: one-sided sign versions

Suppose is a function defined at a point .

Then, we have the following:

Continuity and differentiability assumption | Hypothesis on sign of derivative | Conclusion |
---|---|---|

is left continuous at and differentiable on the immediate left of |
is positive (respectively, nonnegative) for to the immediate left of (i.e., for for sufficiently small ) | has a strict local maximum from the left at , i.e., (respectively, has a local maximum from the left at , i.e., ) for to the immediate left of . |

is left continuous at and differentiable on the immediate left of |
is negative (respectively, nonpositive) for to the immediate left of (i.e., for for sufficiently small ) | has a strict local minimum from the left at , i.e., (respectively, has a local minimum from the left at , i.e., ) for to the immediate left of . |

is right continuous at and differentiable on the immediate right of |
is positive (respectively, nonnegative) for to the immediate right of (i.e., for for sufficiently small ) | has a strict local minimum from the right at , i.e., (respectively, has a local minimum from the right at , i.e., ) for to the immediate right of . |

is right continuous at and differentiable on the immediate right of |
is negative (respectively, nonpositive) for to the immediate right of (i.e., for for sufficiently small ) | has a strict local maximum from the right at , i.e., (respectively, has a local maximum from the right at , i.e., ) for to the immediate right of . |

### What the test says: combined sign versions

Suppose is a function defined around a point (i.e., is defined in an open interval containing ) and is continuous at . We do not care whether is differentiable at ; however, the test makes sense only if is differentiable on the immediate left and immediate right of .

Then, we have the following (we list only the *strict* cases in the table below):

Continuity and differentiability assumption | Sign of on immediate left of | Sign of on immediate right of | Conclusion about local minimum, local maximum, or neither |
---|---|---|---|

is continuous at and differentiable on the immediate left and immediate right of | positive | negative | strict local maximum |

is continuous at and differentiable on the immediate left and immediate right of | positive | positive | neither local maximum nor local minimum |

is continuous at and differentiable on the immediate left and immediate right of | negative | negative | neither local maximum nor local minimum |

is continuous at and differentiable on the immediate left and immediate right of | negative | positive | strict local minimum |

If we replace *positive* by *nonnegative* and *negative* by *nonpositive* in the rows corresponding to strict local maximum and strict local minimum, we could potentially lose the *strictness*.

### Relation with critical points

The typical goal of the first derivative test is to determine whether a critical point is a point of local maximum or minimum. Hence, the test is typically applied to critical points. *However, when applying the first derivative test, we do not need to check whether the point in question is a critical point. In other words, if the condition for being a point of local maximum or minimum is satisfied, then the point in question is automatically a critical point and this condition need not be checked separately.*

### Succinct version

Here is a shorter version: at a critical point, if the derivative changes sign from negative to positive (as we go from left to right) then that is a point of local minimum. If the derivative changes sign from positive to negative (as we go from left to right) then it is a point of local maximum.

## Related tests

## Notes

### Points of inflection

### Examples illustrating why the test is not always conclusive

The following problems could occur when applying this test:

- The function is not continuous, or not differentiable, at points to the immediate left or immediate right of the critical point.
- The function is differentiable on the immediate left and immediate right of the critical point. However, the derivative does not have a uniform sign on the immediate left or the immediate right, i.e., it is oscillatory in sign at points arbitrarily close to the critical point.

Here is a picture of a function illustrating (2):